1. Field of the Invention
The present invention relates to a method of manufacturing multi-strength honeycomb core without splicing and to a multi-strength honeycomb structure produced with the multi-strength honeycomb core.
2. Discussion of Background
In recent years, honeycomb core structures have been used for fabricating various devices, such as aircraft structures, vehicle structures, etc. Honeycomb core is produced with varying cell size, density, material, cell configuration, and surface treatments. Honeycomb core is a structural material that combines strength improvement with weight reduction and unique structural advantages, and therefore it is typically used as a light density filler material for aerospace applications. Thin metallic or composite skins are bonded to the surface of the core to make structures with very high stiffness and low weight.
A primary criterion in aerospace design is weight minimization. As a result, aerospace designers often design components with different densities of honeycomb core splice bonded together. Heavy density core details are used in regions where loading is high or fasteners are used. Conversely, low density core is used in regions with lower strength requirements. For example, high-density honeycomb is used for edge reinforcement and fastener inserts in lower density honeycomb structures.
Honeycomb core may be made from various metallic and non-metallic materials, including aluminum, paper, fiberglass, etc., depending on a given application, and it is available in CUE (Core UnExpanded) blocks, CUE slices (cut to specific thickness), and expanded panels. Honeycomb can be machined for inserts, tapers, or contours, for doubler areas, for edge bevels, for periphery cuts, and for adhesive foam splicing. However, since in many of the above-described applications various density honeycomb cores must be mated to provide areas of lower and higher density, manufacturing difficulties exist as will now be described with reference to FIGS. 1-5.
FIG. 1 illustrates a top view, looking down onto a honeycomb structure 100 having a first density core portion 110, a second density core portion 120, and a third density core portion 130. The three core portions 110-130 are joined together to form a single structure with splices S comprising adhesive joints.
In FIG. 2, a side view of the multi-density honeycomb structure 100 taken along line A--A of FIG. 1 is shown having an upper surface 140 and a lower surface 150 being, for example, contoured and flat surfaces, respectively. These surfaces must be typically machined so as to provide proper alignment between the various density core portions 110-130.
FIG. 3 shows a cross-sectional view taken along line B--B of FIG. 1. As shown in FIG. 3, a mismatch M may occur between, for example, the first density core portion 110 and the second density core portion 120 of the contour surface 140 at the splice S due to machining tolerances and movement of core portions 110 and 120 during the bonding.
FIG. 4 is used to illustrate the manufacturing steps of a honeycomb core 200. In FIG. 4, individual layers 230 are bonded with adhesive in a stacked fashion at locations 210 and 220 equally dispersed throughout the layers 230 at different intervals to form the honeycomb core 200. In FIG. 4, the honeycomb core 200 is shown in a compact or block state, also referred to as Core UnExpanded (CUE). While the core is in this state, machining operations (such as slicing, shaping, beveling, contour machining, etc.) are performed, and the CUE core is manufactured to a variety of cell configurations, thicknesses, lengths, and widths. The density of a CUE or expanded core is a product of the thickness of the individual layers and the adhesive spacing on each layer of the core and can be varied by changing the layer thickness and/or adhesive spacing.
FIG. 5 shows the honeycomb core 200 of the FIG. 4 in an expanded state. As shown in FIG. 5, a honeycomb cell 240 is defined by the bonds 210 and 220 and the layers 230. In order to manufacture a honeycomb core of a given density, a CUE core having layers 230 of a given thickness and adhesive bonds 210 and 220 with a given adhesive spacing are provided. The layers 230 and adhesive spacing are chosen so that, when the CUE core is expanded, a honeycomb core of a given density and corresponding strength results.
As previously described, two or more expanded core portions of different densities must be spliced or joined together with an adhesive foam/film to obtain a multi-strength honeycomb structure. The requirement designers impose on honeycomb core requiring splicing/bonding different types of core together poses some difficulties for the manufacturer.
First, it is critical that the adjacent details are bonded tightly together with each cell edge along the splice line penetrating the splice adhesive. Any gap between the core and the adhesive will appear as a void in subsequent non-destructive testing procedures deeming the assembly non-conforming. Additionally, the splice line must typically be equal to or greater in strength than the adjacent cores.
Second, the core is generally treated to enhance bond strength and inhibit corrosion. As a result, sanding or blending adjacent core details to achieve a flush condition at the splice lines is not permitted. To avoid the need to blend, the separate details must first be produced to close tolerance thickness requirements. Subsequently, the bonding operation must insure that the details are held tightly against the bond tool surface.
The general results of the manufacturing method described above are not conducive to obtaining a near perfect matched surface bond/spliced joint as previously discussed. The mismatched surfaces cause irregularities, for example, when a honeycomb structure has a laminated skin surface bonded to it in a complete assembly. Furthermore, the above-described process can result in moisture attraction and migration into the final bonded assembly, further degrading structural performance.